There are two common types of passenger and freight elevators. In traction elevators, a car is suspended on a rope which is entrained over a drive sheave located in the penthouse of the building. The other end of the rope connected to a counterweight, and a motor rotates the sheave so as to move the car up and down the hoistway between floors or landings. The other common type of elevator is a hydraulic elevator.
Hydraulic elevators employ one or more hydraulic jacks, which are used to raise and lower the car between landings. To raise the car, a motor-driven pump supplies pressurized hydraulic fluid to the jack. In order to lower the car, hydraulic fluid in the jack is vented back to a reservoir.
When moving between floors, it is necessary to control accurately the rate and amount of fluid supplied to the jack, so as to provide a smooth ride and such that the car stops level with the floor. In most hydraulic elevators, this is done with a control valve. When the car is commanded to move to a higher floor, such as when a passengers activates a floor button inside the car or at a hall landing, the pump motor is started, and the pump supplies hydraulic fluid to the control valve which, in turn, supplies pressurized fluid to the jack. When a passenger commands the car to move downwardly, the valve vents fluid from the jack back to the reservoir, bypassing the pump.
In both the up and down directions, the valve is used to control the speed and position of the car. When traveling in the up direction, the valve regulates the amount of pressurized fluid supplied by the pump which is delivered to the jack, by venting some or all of the fluid from the pump back to the reservoir. In this manner, the valve controls the car's upward speed and stops the car when it reaches the desired landing. In the downward direction, the valve regulates the rate at which fluid is vented from the jack to the reservoir to do the same thing.
Generally, hydraulic elevator control valves provide for two flow rates, and thereby two speeds. While traveling between floors, the valve operates at a fast speed setting, allowing a preset maximum flow rate. As the car nears the landing, the valve switches to a slower speed setting, thereby reducing the rate of fluid flow to or from the jack as the car nears the landing.
The foregoing elevators generally use a fixed speed a/c motor and a fixed displacement pump. It has, however, been proposed to control the car's movement using a variable speed pump, which offers the advantage of being able to operate the car at smoothly varying speeds rather than the fixed speeds available from a valve. Another option is to use a fixed displacement pump but with a variable speed motor. However, until recent years this would have required the use of a DC motor which, in the case of hydraulic elevators, is cost prohibitive. More recently, however, with advances in VVVF a/c motor controls, low cost, variable speed, a/c motors are available, which can be used, along with a fixed displacement pump, in place of a valve, to control the rate of fluid flow and thereby the speed of the car.
Hydraulic fluid in the jack must be under sufficient pressure to support both the weight of the car and its own weight. Therefore, fluid in the fluid lines is under relatively high pressure as well, and can cause the line, or connections, to break. If an oil line between the valve and jack should breaks, the elevator car may descend very rapidly. It would therefore be desirable to provide a valve that responds in the case of car overspeed adjacent the jack outlet, such that should any problem occur overspeed would be prevented.